Field of the Invention
Present invention relates to a process for recovery of pure components from product mixture of one step dimethyl ether synthesis reactor. More particularly, the present invention relates to a energy efficient and cost effective process for recovery of unconverted synthetic gas lean in CO2, CO2 stream with minimized content of diethyl ether to enhance the process diethyl ether productivity, recovery of dimethyl ether product, and methanol to be either recycled to dimethyl ether synthesis reactor or to be withdrawn as a product and water.
Description of the Related Art
In one step dimethyl ether (DME) production process, synthetic gas is catalytically converted in DME in a single rector with methanol synthesis (2H2+CO═CH3OH), methanol dehydration (2CH3OH═CH3OCH3+H2O) and water gas shift reaction (CO+H2O═CO2). The overall reaction for synthesis gas conversion to DME is: 3H2+3CO═CH3OCH3+CO2. Formation of methanol and DME is limited by the chemical equilibrium. Per pass syngas conversion or productivity of one step DME synthesis reactor will depend on the synthetic gas composition, reactor pressure and temperature significantly due to chemical synergy among reactions involved. It is apparent to understand that higher per pass syngas conversion or productivity of DME synthesis reactor would lead to small size and lower operating cost of the reactor, separation and recycles systems. The product mixture from one step DME synthesis reactor will be a mixture of unconverted synthetic gas, DME, CO2, methanol and water.
The separation systems for production of pure DME and DME rich stream from the product mixture of one step DME reactor have been disclosed in published literature [U.S. Pat. No. 5,908,963; Pat. No. U.S. Pat. No. 6,458,856 B1; Pat. No. US2010/0216897; Pat. No. U.S. Pat. No. 7,910,630 B2; Pat No. EP2070905A1; Pat. No U.S. Pat. No. 7,652,176 B2; Pat. No. U.S. Pat. No. 8,835,517 B2].
U.S. Pat. No. 5,908,963 discloses a process for preparation of fuel grade dimethyl ether (DME). In this patent unconverted synthetic gas is separated from other compounds using condensation and methanol absorption based separation system. The recovered synthetic gas is recycled to DME synthesis reactor. The major focus is given on production of fuel grade DME containing significant amount of methanol. The removal of CO2 from synthesis gas is not mentioned in the patent.
U.S. Pat. No. 6,458,856 B1 discloses a process for one step production of dimethyl ether (DME) from synthetic gas. The DME reactor effluent is separated into vapor mixture comprising DME, CO2 and unconverted synthetic gas and liquid stream comprises of DME, methanol and CO2 using high pressure flash. The vapor mixture is scrubbed with the scrubbing solvent comprising methanol and DME mixture to remove the CO2. The recovered synthetic gas lean in CO2 lean is recycled to DME synthesis reactor. DME, methanol, CO2, and water were recovered from the liquid mixture using number of flash and distillation column. Invention claims the novelty for improvement in reduction of CO2 and DME concentration in recycled synthetic gas achieved by using DME and methanol mixture as scrubbing solvent in comparison to pure DME as scrubbing solvent.
Pat. No. US2010/0216897 discloses a process where pure DME and methanol were used as scrubbing solvents in two separate zones of the absorption column to recover the CO2 lean synthetic gas. It was mentioned that invention utilizes the higher solubility of CO2 in DME to minimize the quantity requirement of scrubbing solvent for reducing the CO2 concentration to desired level and utilizes the lower vapor pressure of methanol to minimize the loss of scrubbing solvent (methanol+DME) in recovered synthetic gas to be recycled to DME synthesis reactor.
U.S. Pat. No. 7,910,630 B2 discloses a method in which methanol and water mixture from DME reactor effluent is removed using condensation. A cooled solvent of dialkyl ether of a polyethelene glycol was used to scrub the CO2 and DME from uncondensed syngas. The scrubbed gas is recycled to DME reactor. The CO2 and DME are selectively desorbed in the subsequent steps.
Pat. No. EP 2070905A1 discloses a process in which a product mixture from DME reactor is scrubbed with a liquid solvent being rich in dialkyl ether of a polyalkylene glycol. The dissolved CO2 and DME in solvent are separated using the distillation columns. Condensed methanol water mixture is either recycled to DME synthesis reactor or passing to methanol dehydration reactor. No separation of condensed methanol water mixture is disclosed. The one of ordinary skilled in the art can understand that for stable operation water draw off from process is required to maintain the water concentration in the process.
U.S. Pat. No. 7,652,176 B2 assigned to Haldor Topsoe discloses a process in which product mixture obtained from the DME reactor is scrubbed with a liquid solvent being rich in potassium carbonate or amine for selective absorption of carbon dioxide in liquid solvent to reduce the CO2 in the recycled unconverted synthetic gas. An additional solid adsorbent comprised of Zeolite, molecular sieves or activated alumina is used to further reduce the CO2 concentration in the recycled unconverted synthetic gas. Subsequently, distillations columns were used to separate the solvent, DME, CO2 and methanol from the mixture thereof.
U.S. Pat. No. 8,835,517 B2 discloses a method that uses chilled water for scrubbing the DME and methanol from DME reactor product mixture in high pressure absorption column. Mixture of water, DME, CO2 is preheated in a heat exchanger and flashed in lower pressure column. A DME rich stream is generated from this flash column. The unconverted syngas produced is lean in DME and CO2. However, the concentration of dimethyl ether in DME rich stream and concentration of DME and CO2 in recovered unconverted synthetic gas are not disclosed. Moreover, CO2 removal from DME and water is also not addresses.
It is important to note that methods disclosed in the prior art for DME production using one step DME synthesis rector are focused on; reducing the concentration of CO2 in unconverted synthetic gas using solvents like DME, methanol, chilled water, mixture thereof, dialkyl ether, potassium carbonate or amine rich solvents; minimizing the concentration of scrubbing solvent in unconverted synthetic gas to be recycled to the DME synthesis reactor. Pat. No. US2010/0216897 describes that how application of pure DME and methanol in two separate zone of absorption column is more effective in terms of requirement of less quantity of scrubbing solvent, lower concentration of CO2 in recovered unconverted synthetic gas and less loss of DME and methanol with the recovered unconverted synthetic gas in comparison to using the mixture of methanol and DME in single zone.
It is vital to note that application of absorption process either with dimethyl ether or menthol or their mixture as a scrubbing solvent for recovery of unconverted synthetic gas lean in CO2 from one step DME reactor effluent do not require the investments for purchase of solvent, solvent storage and solvent regeneration facilities because these facilities are inbuilt/exists in the process. Therefore, application of DME and menthol and mixture thereof as solvent seems more attractive option over to the solvents which are not generated in situ in the process like dialkyl ether, potassium carbonate and amine rich solvents. Moreover, the solvents other than DME, methanol and water may also lead the contamination of DME. The contaminated DME can also adversely affect the system where it will be used.
The prior arts documents which disclose the application of DME and menthol and mixture thereof as a solvent to produce pure DME uses number of separation steps to meet the objective for recovery of pure component from product mixture of one step DME reactor. For example; U.S. Pat. No. 6,458,856 B1 teaches the production of pure DME (FIG. 1) involves 7 separating columns and one DME dehydration reactor to separate the syngas, CO2, DME, methanol, and water. In FIG. 2 of this patent (U.S. Pat. No. 6,458,856 B1) water and methanol discharge points from the process are not stated. This implies that water and methanol buildup in the process will increase with the passes of the time and process will become unstable in long run and process scheme is not feasible to implement at actual site of operation. Pat. No. US2010/0216897 disclosing a process shown in FIG. 6 to separate CO2, DME, methanol, and water from DME reactor outlet streams includes 9 separating column.
The person of ordinary skill in the art can understand that CO2 separation from DME, methanol and water mixture using distillation column will require refrigeration or cooling media having temperature much below the room temperature in its condenser for cooling the vapor of this column to the lower temperature to avoid loss of DME product with recovered CO2 stream. This is attributed to high solubility of CO2 in DME, low boiling point value of −56.6° C. for CO2 and requirement of almost complete removal of CO2 from DME, methanol and water mixture to avoid the contamination of DME product with CO2. The quantitative effect of cooling media temperature on DME loss in recovered CO2 stream is illustrated in example 1 of the present invention.
It is essential to note that DME production in two step process wherein methanol is synthesized in one reactor and methanol dehydration is carried out in subsequent second reactor. Methanol synthesis reactor operates at pressure in the range of 70-90 bars and needs huge compression for synthetic gas compression. In one step DME production process, DME synthesis reactor operates in the significantly lower pressure range of 40-60 bars due to enhanced driving force for DME synthesis by simultaneous removal of methanol from system. In two steps DME production process, compression cost is more than 60% of overall process cost. It is clear that compression cost of synthetic gas in one step process will be much lower than the two step process.
However, process installation and operational cost for recovery of components from product mixture (methanol, unconverted synthetic gas) of methanol synthesis reactor and recovery of components from product mixture (methanol, DME and water) from methanol dehydration reactor is much easier than the recovery of components from single step DME reactor product mixture comprising unconverted synthetic gas, DME, CO2, methanol and water in one step process. The complexity in one step process arises due to requirement of recovered unconverted synthetic gas with significant reduced concentration of CO2, presence of highly volatile DME in product mixture which tend to loss with unconverted synthetic gas, need of refrigeration facilities to generate the cooling medium to be used in the condenser of CO2 separation distillation column for minimizing the loss of DME with recovered CO2 stream.
The methods reported in prior art for separation of DME, CO2, methanol, water, and unconverted synthetic gas from product mixture of one step DME reactor are complicated and capital intensive due to involvement of number of separation steps and expensive refrigeration facilities. Hence, there is a need for a more simpler and cost effective process for separation of DME, CO2, methanol, water, and unconverted synthetic gas from product mixture of one step DME reactor. Therefore, to get the real fruit of less compression requirement and higher per pass conversion of synthetic gas in single DME synthesis reactor and no need of refrigeration facilities in comparison to two step process, a separation process which can simplify the separation scheme to reduce the number of separating steps and can eliminate the need of refrigeration which includes number of equipment in its generation to make the process cost effective and to miniaturize the DME production plant so as it can be used as a modular process will be of great importance.